Ordinary lumber is a major article of commerce. It is of value because of a number of physical attributes, not the least of which is resistance to bending stress.
Resistance to bending stress is evaluated by a measurement known as Modulus of Rupture (MOR) which in turn depends upon (1) compressive strength in the area being compressed, i.e. the uppermost portion of cross-section in the middle of a horizontal, top-loaded, end-supported beam, (2) shear strength of the center area, top-to-bottom, of the top-loaded beam, and (3) most importantly, tensile strength of the lower, tensile-stressed mid-length area of the top-loaded, end-supported beam.
Resistance to bending stress in ordinary lumber is greatly affected, negatively, by natural defects such as knots, checks, wane, pitch pockets, etc. However, deficiency in MOR is the primary reason that consolidated comminuted woody products are not now a satisfactory substitute for lumber in the vast majority of lumber uses, notwithstanding the problems encountered in the use of natural wood lumber.
Another important physical property of lumber is its resistance to wear and abrasion, in turn also affected by defects, species, density, and growth rings.
Also important in certain use applications is resistance to deterioration in service due to exposure to mold, moss, insects and microorganisms. Lumber of some species, such as cedar, exhibit natural resistance but fall short in uses involving extreme exposure such as subterranean or marine contact. Consolidated comminuted wood can easily be fully treated with preservatives and attain uniform distribution.
And further, in end-use applications where fire-retardance of the article is important, retardants may be easily added to the comminuted wood furnish before consolidating, thus obviating the conventional autoclave pressure treatment of lumber with retardant solution then drying. The benign nontoxic nature of boric acid retardant cannot be used to impregnate lumber because of its low water solubility, but it can be added to particulate wood.
Composite lumber containing creosote preservative, that is suitable for laminating into articles (e.g. railroad ties) can accomplish objectives of both ecology and economy by using substantial percentages of recycled old ties whose creosote content prevents easy disposal. The old ties may be hammermilled and the particles metered into new resin-furnish and thereafter compressed and the resin cured. A product of high density is produced with much superior and long lasting spike-holding strength and plate-cut resistance. The absence of flaws, checks and knots prevents the incursion of water and rot to the interior and will give a much longer useful life than that of solid wood.
Another sometimes troubling shortcoming in the use of natural lumber is the occurrence of warping and/or checking, each due to uneven shrinkage upon drying along the radial growth ring direction compared to the circumferential direction. This effect is absent in consolidated articles.
Accordingly, it is an object of the invention to provide a process for economically producing wood products of structural lumber dimensions from comminuted wood.
More particularly, it is an object of the invention to provide wood products of structural lumber dimensions formed from comminuted wood and which products have physical characteristics sufficient to permit the products to be utilized in place of natural wood lumber.
Another object is to provide improved apparatus for forming structural lumber sized products from comminuted wood particles.
Other objects and advantages will become more apparent hereinafter.